The present invention relates to transformer cores. More particularly to transformer cores made from strip or ribbon composed of ferromagnetic material, especially amorphous metal alloys.
Transformers conventionally used in distribution, industrial, power, and dry-type applications are typically of the wound or stack-core variety. Wound core transformers are generally utilized in high volume applications, such as distribution transformers, since the wound core design is conducive to automated, mass production manufacturing techniques. Equipment has been developed to wind a ferromagnetic core strip around and through the window of a pre-formed, multiple turns coil to produce a core and coil assembly. However, the most common manufacturing procedure involves winding or stacking the core independently of the pre-formed coils with which the core will ultimately be linked. The latter arrangement requires that the core be formed with one joint for wound core and multiple joints for stack core. Core laminations are separated at those joints to open the core, thereby permitting its insertion into the coil window(s). The core is then closed to remake the joint. This procedure is commonly referred to as xe2x80x9clacingxe2x80x9d the core with a coil.
A typical process for manufacturing a wound core composed of amorphous metal consists of the following steps: ribbon winding, lamination cutting, lamination stacking, strip wrapping, annealing, and core edge finishing. The amorphous metal core manufacturing process, including ribbon winding, lamination cutting, lamination stacking, and strip wrapping is described in U.S. Pat. Nos. 5,285,565; 5,327,806; 5,063,654; 5,528,817; 5,329,270; and 5,155,899.
A finished core typically has a rectangular shape with the joint window in one end yoke. The core legs are rigid and the joint can be opened for coil insertion. Amorphous laminations have a thinness of about 0.025 mm. This causes the core manufacturing process of wound amorphous metal cores to be relatively complex, as compared with manufacture of cores wound from transformer steel material composed of cold rolled grain oriented (SiFe). The consistency in quality of the process used to form the core from its annulus shape into rectangular shape is greatly dependent on the amorphous metal lamination stack factor, since the joint overlaps need to match properly from one end of the lamination stack factor, since the joint overlaps need to match properly from one end of the lamination to the other end in the xe2x80x98stair-stepxe2x80x99 fashion. If the core forming process is not carried out properly, the core can be over-stressed in the core leg and corner sections during the strip wrapping and core forming processes which will negatively affect the core loss and exciting power properties of the finished core. Core-coil configurations conventionally used in single phase amorphous metal transformers are: core type, comprising one core, two core limbs, and two coils; shell type, comprising two cores, three core limbs, and one coil. Three phase amorphous metal transformer, generally use core-coil configurations of the following types: four cores, five core limbs, and three coils; three cores, three core limbs, and three coils. In each of these configurations, the cores have to be assembled together to align the limbs and ensure that the coils can be inserted with proper clearances. Depending on the size of the transformer, a matrix of multiple cores of the same sizes can be assembled together for larger kVA sizes. The alignment process of the cores"" limbs for coil insertion can be relatively complex. Furthermore, in aligning the multiple core limbs, the procedure utilized exerts additional stress on the cores as each core limb is flexed and bent into position. This additional stress tends to increase the core loss resulting in the completed transformer.
The core lamination is brittle from the annealing process and requires extra care, time, and special equipment to open and close the core joints in the transformer assembly process. Lamination breakage and flaking is not readily avoidable during this process opening and closing the core joint. Containment methods are required to ensure that the broken flakes do not enter into the coils and create potential short circuit conditions. Stresses induced on the laminations during opening and closing of the core joints oftentimes causes a permanent increase of the core loss and exciting power in the completed transformer. These technical concerns are particularly relevant wherein large annealed wound amorphous metal transformer cores, such as those used in large power transformers (typically distinguished as having a duty rating of at least 500 KVA) are to be produced. The mass of such transformer cores very often deleteriously affects the handling of large annealed wound amorphous metal transformer cores during the assembly process of both the core itself, as well as of the transformer in which the core is utilized. Further the mass of such transformer cores also frequently compounds the likelihood of flaking, cracking or breaking of the embrittled annealed amorphous metal cores which leads to increased potential for greater core losses in the finally assembled transformer. In such applications operating efficiency is of paramount importance and such cracks or breaks in the annealed amorphous metal decreases the operating efficiency of the core. Flaking, wherein pieces of the core are broken and separated, usually find themselves trapped in between the laminar layers of the wound core and decrease stacking efficacy, as well as raise the likelihood of causing electrical short circuits. This too results in core losses and decreased operating efficacy. Flaking is also deleterious when the core is to be used in a fluid filled, i.e., oil filled transformer. In addition to the likelihood of core losses due to decreased stacking efficacy, the loose flakes which may be present in the fluid also lower the dielectric strength of the liquid and also reduce the operating efficacy of the core.
A further inherent limitation of such annealed wound amorphous metal transformer cores is that when they are oriented in a vertical position, as is typical in most transformer designs, the mass of such annealed wound amorphous metal transformer cores may crack under its own weight. While weight distribution of annealed wound amorphous metal transformer cores is more evenly distributed amongst laminar layers when in a horizontal position, once uprighted and oriented vertically the xe2x80x9csaggingxe2x80x9d of the annealed wound amorphous metal transformer cores may cause cracking.
Accordingly there exists a real and present need for improvements to annealed wound amorphous metal transformer cores and assemblies which address and overcome one or more of these shortcomings.
It is to these and other shortcomings that the present invention is directed.